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    TIẾN SĨ Shear failure mechanisms of segmental prestressed concrete beams with external tendons

    Quy Ẩn Giang Hồ Quy Ẩn Giang Hồ Đang Ngoại tuyến (956 tài liệu)
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  6. Shear failure mechanisms of segmental prestressed concrete beams with external tendons

    ABSTRACT
    Recently, precast segmental concrete structure with external tendons combined with
    span-by-span construction method is one of the innovative structures and methods in bridge
    construction, because of substantial cost and time saving in construction. The comprehension
    of the shear behavior of segmental concrete beams with external tendons, therefore, is
    required. In addition, the prediction of the shear carrying capacity is an important part in
    design such structure. In the segmental concrete structure, a segmental joint represents the
    location of discontinuity of non-prestressed steel reinforcement. The section of segmental
    joint can be weaker to compare with other sections in segmental concrete beams. The shear
    behavior and the shear carrying capacity of segmental concrete beams, therefore, are strongly
    affected by the shear transfer mechanism of segmental joint.
    In order to investigate the shear behavior and shear carrying capacity of segmental
    concrete beams, many studies were concentrated on the male-female shear key to evaluate the
    shear transfer across a segmental joint. The studies on the male-female shear key seem to be
    suitable for the segmental joints close to the supports in the simply supported beams where
    the joint opening is insignificant. Due to the effect of moment, segmental joint may open. The
    compression force and the shear force are complexly transferred across an opened segmental
    joint. The effect of the shear transfer mechanism across an opened segmental joint on shear
    behavior and shear carrying capacity of segmental concrete beams with external tendons is
    necessary.
    In this thesis, experimental and analytical investigations are conducted to examine the
    effect of joint opening on shear transfer mechanism of segmental joint, the shear behavior and
    shear carrying capacity of segmental concrete beams with external tendons. The simplified
    method, the truss model, with a high accuracy is then proposed to predict the shear carrying
    capacity of segmental concrete beams with external tendons.
    In the first set of experimental investigation, the effects of joint position, prestress level
    and joint type, i.e. epoxy joint and dry joint were carried out. The experimental results showed
    that the shear behavior and shear carrying capacity of segmental concrete beams with straight
    external tendons were strongly affected by a local behavior of keyed segmental joint. The
    local behavior of keyed segmental joint, however, was influenced by joint position and
    prestress level. Joint opening and height of joint opening increased when joint position moved
    close to the loading point and prestress level decreased. The height of joint opening affected
    the contact area of segmental joint which resulted in the change of the compression strut
    transferring across an opened segmental joint. As joint position moved close to the loading
    point the shear failure mode changed from the shear compression failure mode to the flexural
    iii
    shear failure mode. As prestress level decreased the shear failure mode changed from the
    shear compression failure mode to the diagonal tension failure mode. With the same tendency
    of joint position and prestress level, the shear carrying capacity of segmental concrete beams
    decreased. At the ultimate stage, the shear transfer mechanism and shear carrying capacity of
    segmental concrete beams with dry joint was similar to that with epoxy joint; even through
    the concrete stress at the lower fiber was about 6 N/mm2.
    Analytical investigation by finite element method (FEM) for segmental concrete beams
    with straight external tendons showed that the compression stress flow transferred across an
    opened segmental joint were found to have two tendencies and affected by joint position.
    These two tendencies of the compression stress flow from the loading point and its thickness
    were then evaluated by formulae. The modified model for segmental concrete beams with
    straight external tendons had been proposed based on the simplified truss model. The shear
    carrying capacity assessed from the modified model had provided a better accuracy in
    comparison with the existing prediction models for segmental concrete beams with straight
    external tendons. The collected experimental results were also compared to confirm the
    applicability of the modified model in the wider range of segmental concrete beams such as
    bonded or unbonded internal tendons.
    In the second set of experiment, the contribution of draped tendons on the shear behavior
    and the shear carrying capacity of segmental concrete beams with external tendons were
    examined. It was found that deviator force and local behavior of segmental joint were affected
    the shear behavior and shear carrying capacity of such segmental concrete beams. FEM
    results showed not only the deviator force and the local behavior of the segmental joint but
    also the transfer mechanism of prestressing force from an anchorage which governed the
    compression stress flows in the shear span. The modified model was extended to predict the
    shear carrying capacity of segmental concrete beams with draped external tendons when an
    inclined angle of draped external tendons was larger than 2 deg. When an inclined angle of
    draped external tendons was smaller than 2 deg., the modified model for segmental concrete
    beams with straight external tendons could be applied with considering the effect of small
    inclined angle. The shear carrying capacity predicted from the extended modified model for
    draped tendons showed a good accuracy in comparison with the existing prediction models
    for segmental concrete beams with draped external tendons.
    The shear behavior of segmental concrete beams with external tendons in this study was
    examined. The effects of joint position, prestress level and joint type, deviator force and
    transfer mechanism of prestressing force from an anchorage were evaluated. The good
    prediction of the shear carrying capacity of segmental concrete beams with external tendons
    from the proposed simplified models was obtained by comparing to the existing models.

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